Moving resonator

I decided to build just one recording room, lets say semi live, and like to do some extra absorption for vocal and some sensitive acoustic instruments. Must admit that vocal both looks pretty claustrophobic, so decided to go with some kind of moving resonators. I planned to make them for band from 200 hz-1 khz :roll: Here's first and the second drawing... Not sure is this ok, so please give some comment :D Also, did somebody checked which slot, slat (or whatever) excel calculator works :?:

The lower pic looks like it would either break where the wheels attach or fall over, not sure how practical that would be - neither will get you into the frequency range you want with those slat/slot spacings (a minor point I know) For a range of 200-870 hZ (approximately) you would (for the top design) want slat/slot spacings of 25/150mm using 19mm thick slats - your depth would likely run from 25mm to 500mm. One problem with variable depth helmholtz is that they won't exhibit the same properties at other than perpendicular incidence, unless you build individual separators between each slat so you have several small, INDIVIDUAL Helmholtz resonators. This makes construction more difficult but you won't get as consistent results off-axis otherwise. Calculators - John has yet to get complaints on slat resonator workings even BEFORE the web-wide "formula error" was detected and corrected here and at Studiotips - While Will (interface) is much more mathmatically adept than I am, his formula is accurate for EMPTY helmholtz traps. There will be some shifting of frequency with differing fill materials, and it doesn't look(to me, anyway) like a practical approach can be had that will properly predict exactly HOW this shift will affect results with differing materials. From what I can see, no two companies' 2" product, even at the SAME DENSITY, would have the same acoustic resistance so there doesn't seem to be a way to even GUESS what this effect will be, other than it will be different. Especially since no two companies (or even different products within the SAME company) are using the same specs across the board. It seems to me that a person would need to set up a way to test for acoustic resistance themselves in order to use this as a calculation tool. The formula used in our calculator is based on F. Alton Everest's published results, and is intended for half-fill with typical 3 PCF insulation with a cloth covering, all placed AGAINST the front slats - Will's calculator (interface) is designed for NO FILL but just specific acoustic resistance cloth placed immediately behind the slats. Not very many products even SPECIFY acoustic resistance, only absorption at a limited range of frequencies... Both calculators take into account hole depth correction in some way, and calculation results aren't very far apart, musically speaking - (less than an octave by far) - So, especially in YOUR case (where you're trying to create a broadband trap ANYWAY) I don't see the need to obsess over it any further. Realistically (from my situation) it will probably be YEARS before I get time (if then) to experiment personally with this, so if you still have doubts you should either abandon them, or set up a way to test a few different designs and compare actual results to calculated values. I'm not trying to be difficult here, only a practical assessment of my own capabilities/time... Steve

Thanks Steve, I agree, the second one looks unstable. I hope that i understood you well, so here's the modification of 1st. About calculators, I'm sure if works for John, will work for me too. So, corrected RSD calculator... :wink:

Fine, except you'll want at least two inches (50mm) of insulation immediately behind the cloth in each chamber, for smoothest Helmholtz action... Steve

Thanks Steve, I will post pictures and impressions, now I'm gonna do some "woodcraft" :wink:

moby wrote:

Thanks Steve, I agree, the second one looks unstable. I hope that i understood you well, so here's the modification of 1st. About calculators, I'm sure if works for John, will work for me too. So, corrected RSD calculator... :wink:

I do think that the formula given at RSD is a “thumb x air” formula. Rod Gervais asked a very good question some time ago at studiotips.

Rod Gervais wrote:

I understand the principle - but still feel that if something does not establish a method of obtaining the correct stiffness for the covering slats - then the entire results wind up skewed - and not at all what one would expect. I don't look to you for a calculation - but again - an example - If i build a box - 8' x 8' x 4" in depth - with attachments at 2'-0" o.c. - 2" air space - 2" 6psf rockwool - and then cover it with 1x4 pine boards with a one inch slot....... the resultant absorbtion frequency would be 537 hz If I pull out the 2' framing - and only attach the 8' members at the 2 edges - the resultant frequency would be exactly the same (by the calculation method) even though the amount of movement allowed in the surface becomes greatly increased. It seems that in order for this to really calculate accurately (otherwise (IMHO) it's pretty useless) you would have to establish the maximum flex of the slot member - and for that you would need to include a query about the member properties - and then calculate the maximum distance between fastening members. Or am I mistaken - and the 2 different systems would in fact operate exactly the same? Once again - I know the math for this would become very VERY complicated - and I am not looking for a solution - just the concept of what it is I am describing to you.

More details can be read at http://forum.studiotips.com/viewtopic.php?t=1513 However, the second Excel sheet which I wrote considers all kind of boundary conditions. E.g.: 1. Slot/Slat properties, this includes: density of slat / thickness of slat / width of slot/ width of slat. 2. Flow resistance of absorber at neck (e.g. fleece material) 3. Porous absorber behind the slotted plate 4. Air cavity The Slot/Slat properties will be elaborated soon (stiffness, loss factor…) This can be found in the topic Wrong Helmholtz formula!? http://www.johnlsayers.com/phpBB2/viewt ... 2&start=15 Figure 6.18, on page 178 in the Book “Acoustic Absorbers and Diffusors / Theory, Design and Application” demonstrates how well the predicted and measured absorption coefficients fit together. Uno Ingard did a lot of researches in 1953 about acoustic resonators. “On the Theory and Design of Acoustic Resonators. (JASA, 1953, Volume 25, Number 6).” More details can also be found in many excellent books, e.g.:Theoretical Acoustics (Morse and Ingard), Schallabsorber (Mechel), Acoustic Absorbers and Diffusors (Cox and D’Antonio), Noise and Vibration Control Egineering (Leo L. Beranek), Principles and Applications of Room Acoustics (Cremer) and many more… I couldn't find the RSD formula anywhere and scientifically it doesn't make any sense to me!!! Since the air cavity is split right at the neck of the slotted plate, none of any mentioned “inner end correction” can be applied for the model of moby!

Interface, yes that's the same question what i try to ask, but i tried to draw... I agree, looks too easy, and i must admit that I'm bit confused with the upper side of every slat. It's touching the "separator" and must do something with mass of slat :?: :?: I'm still thinking about and i mentioned John "real experience in the environment" like reason to jump in some crafting based on the RSD corrected calculator. maybe slat/slot is not the best idea, maybe Perforated Panel with Porous calculator works better for my "moving thing" :?: I really don't know, but will be happy to find out. Sure most of RSD members will be happy too :D Calculating don't take too much time, especially for good mathematicians (I'm not :? ), but crafting of few "bad slatted closet" may take good portion of energy. So, will be nice is somebody can figure right calc or maybe measure some finished... I hate to say, but only i can do is to make some based on the words of experienced guys :cry:

moby wrote:

So, will be nice if somebody can figure right calc or maybe measure some finished...

Moby, did you check the 2nd calculator??? If yes? What's missing?

Moby, did you check the 2nd calculator???

Not sure, 2nd is "Slit Resonator+Fabric+Wool+AirEN"? From my knowledge guys stopped to comment about. I would like to know did somebody tried to craft something based on 2nd calc. From theoretical side of view seems OK, but as i mentioned I'm not an math expert... Or I'm missing something :?:

moby wrote:

Moby, did you check the 2nd calculator???

Not sure, 2nd is "Slit Resonator+Fabric+Wool+AirEN"? From my knowledge guys stopped to comment about. I would like to know did somebody tried to craft something based on 2nd calc. From theoretical side of view seems OK, but as i mentioned I'm not an math expert... Or I'm missing something :?:

You don’t need to be an expert in math to compare curves. Try to find some measured result’s. Measurements which are done in the Kundt’s tube (or impedance tube) will be more accurate. If possible check the books and articles which I’ve mentioned as well. They will fit! Good luck!!!

Thanks Interface :D Guys, any comments? :idea:

OK, i can try, but i need some advice about box construction :? Really don't know how to place slat. Or maybe to do with one slat and two slats per box :?:

Since the air cavity is split right at the neck of the slotted plate, none of any mentioned “inner end correction” can be applied for the model of moby

OK, i will ask directly :wink: Where to put separators? Interface, can you make some drawing, maybe will be easier to figure point. BTW i don't have source for books you mention, and have to ask you. Thanks

moby wrote:

Since the air cavity is split right at the neck of the slotted plate, none of any mentioned “inner end correction” can be applied for the model of moby

OK, i will ask directly :wink: Where to put separators? Interface, can you make some drawing, maybe will be easier to figure point. BTW i don't have source for books you mention, and have to ask you. Thanks

Why do you need separators at all? Can't you do it without?

Of course i can! Why not :?: (to try) :? I was little confused about this, but...

One problem with variable depth helmholtz is that they won't exhibit the same properties at other than perpendicular incidence, unless you build individual separators between each slat so you have several small, INDIVIDUAL Helmholtz resonators.

You want to say that 1st version of resonator can do the job (of course to calculate correct slat/slot/tick) :?:

moby wrote:

Of course i can! Why not :?: (to try) :? I was little confused about this, but...

One problem with variable depth helmholtz is that they won't exhibit the same properties at other than perpendicular incidence, unless you build individual separators between each slat so you have several small, INDIVIDUAL Helmholtz resonators.

You want to say that 1st version of resonator can do the job (of course to calculate correct slat/slot/tick) :?:

The oscillating air in the resonator mouth is the mass. The mass doesn’t change in order to your drawings. The air cavity behind the slots represents the spring. It can be also seen as: the part volume behind each mass represents a spring. The stiffness of the spring becomes softer if you increase the air cavity and vice versa. What you get is a parallel connection of different spring stiffness anyway. So you don't need to separate the volumes.

So you don't need to separate the volumes

Sounds great :D So if I don't try i will never know how that really sounds :roll: Thanks Interface, and thanks for drawing :wink:

So, moby, you promised some pix and stuff... dunno if yer still around, but it looked like a funny plan... so, bring it on!

Yes, I forgot to do that :oops: Resonator works like charm. Will post the pic's soon :wink:

Here are the pic's :D I included the .skp if you want to craft :wink: Feel free to ask about details.

cool. Dunno if I like the Green colour but for the rest nice job and thanx for the pix!

Ro wrote:

cool. Dunno if I like the Green colour but for the rest nice job and thanx for the pix!

If you wanna ride, don't ride the white horse. That's why I choose green :wink:

What is the practical comparison of this type of "gobo" or baffle vs. a regular "gobo" or baffle. What does this moving resonator do to the sound in the room as opposed to using basic OC 703? Besides the fact it looks really cool... Thanks!